FIG. 1 illustrates an exploded view of a directional antenna 100 from U.S. Pat. No. 5,191,349 “Apparatus and method for an amplitude monopulse directional antenna”, which is incorporated by reference herein in its entirety. Directional antenna 100 includes a radome assembly 101, a ground plate assembly 102, a base plate 103, and an adapter plate 104. The radome 105 is manufactured from a polyethersulfone resin having various structures formed on an interior surface, including fastening posts 106, internally threaded grounded portions 107 of the monopole antenna elements, and free portions 108 of the monopole antenna elements. Monopole antenna portions 107 and 108 are coated with copper directly on the surfaces thereof. Capacitors 109 are formed directly on the interior surface of radome 105. Upon assembly, copper coated antenna portions 107 and 108 contact capacitors 109 to form folded monopole antenna elements. Ground plate assembly 102 includes a conducting plate 110. A beam forming network (BFN) is formed on a circuit assembly 111, which is described in detail in the previously mentioned '349 patent. Circuit assembly 111 includes passages 112 providing clearance for free antenna elements 108 to extend through conducting plate 110 and through circuit assembly 111. Connectors 113 electrically couple the aircraft processing and signal generating apparatus (not shown) to the BFN on circuit card assembly 111.
The main disadvantage of this amplitude monopulse directional antenna for use in a communications system configured for Traffic Collision and Aviodance System (TCAS), Transponder, and Universal Access Transceiver (UAT) communications is the difficulties of the omnidirectional mode because of the amplitude and phase differences of the transmission path between the channels and between antenna cables. To eliminate these errors, a special calibration network with variable phase shifters, phase detectors, and an additional calibration signal source is required. Another disadvantage of this antenna is low efficiency due to the strong mutual coupling between the diagonal and adjacent inputs/outputs of the BFN. Experimental testing of the directional antenna disclosed in the '349 patent show 4.1 dB parasitic coupling between diagonal input/output ports at 1090 MHz, and therefore only 61% efficiency. The strong parasitic coupling can be explained by the strong mutual coupling between antenna monopoles and by poor matching between the BFN and the antenna monopoles because the matching network described in the above patent is a one-step quarter wavelength transformer with a narrow frequency band. Combining the narrow-band transformer and the narrow-band 4×4 hybrid matrix with two-branch 90-degree hybrids causes the total narrow band frequency range of the prototype antenna. Therefore, this antenna is not acceptable for the high efficiency combined TCAS/Transponder/UAT system with a frequency range of 978 MHz-1090 MHz or 10.8% bandwidth. A further disadvantage of antenna 100 is that it consists of three metal plates: ground plate assembly 102, base plate 103, and adapter plate 104. The three metal plates therefore mean the antenna is bulky, heavy, and expensive to build.
It is therefore an object of the invention to provide a directional/omnidirectional antenna arrangement having improved performance over existing aircraft antennas while solving the attendant problems of known antennas.
It is another object of the invention to provide a directional/omnidirectional antenna system that minimizes the amount of base metal plates required therefor.
A feature of the invention is an array of folded monopoles coupled to a switched beam forming network.
Another feature of the invention is the feeding and shorting elements associated with each of the folded monopoles are strips formed on a hollow cylindrical dielectric element.
An advantage of the invention is that the number of metal base plates in the antenna system is minimized.
Another advantage is that the size and weight of the antenna system is minimized.